Deaminoneuraminic acid (3-deoxy-D-glycero-D-galacto-nonulosonic acid or 2-keto-3-deoxy-D-glycero-D-galacto-nononic acid; hereinafter referred to as "KDN") has the same structure as that of sialic acid except that N-acyl group linked to 5-position carbon of sialic acid is replaced with hydroxyl group. It has been heretofore revealed that KDN widely ranges over the living world, as a constitutional component of complex carbohydrate, and has a variety of existing forms, in the same manner as sialic acid. On the other hand, KDN has unique properties different from those of sialic acid. For example, it has been clarified that KDN-containing complex carbohydrate plays an important role in the ovum-sperm interaction during fertilization. A great interest is attracted by elucidation of structure and function of KDN-containing glycoprotein and glycolipid.
By the way, those known as an enzyme to cleave the KDN ketosidic linkage of KDN-containing complex carbohydrate (deaminoneuraminidase; hereinafter referred to as "KDNase", if necessary) include KDN-sialidase existing in liver of loach (Li, Y. -T. et al. Archives of Biochemistry and Biophysics, Vol. 310, No. 1, pp. 243-246 (1994)). The present inventors have found that an enzyme having a sialidase activity and a deaminoneuraminidase activity exists in a tissue such as ovary of fish such as rainbow trout (Angata, T. et al., Glycobiology, Vol. 4, pp. 517-523 (1994)).
However, any of the foregoing enzymes does not specifically cleave the KDN ketosidic linkage. Some of the foregoing enzymes involve a sialidase activity to a degree approximately the same as the activity to cleave the KDN ketosidic linkage (enzyme originating from loach described above), and others involve a sialidase activity stronger than the activity to cleave the KDN ketosidic linkage (enzyme originating from rainbow trout described above). No enzyme has been known, which specifically acts on only KDN. The enzyme originating from loach is known to have an optimum pH of about 4.5. The present inventors have found that the enzyme originating from rainbow trout has an optimum pH of about 4.4.
It is known that the enzyme originating from loach has, in the vicinity of pH 6, an amount of the deaminoneuraminidase activity which is 65% of an amount of the deaminoneuraminidase activity obtained at its optimum pH. On the other hand, the present inventors have found that no deaminoneuraminidase activity is found in the vicinity of pH 6.5 in the case of the enzyme originating from rainbow trout. Therefore, it has been difficult to make a deaminoneuraminidase reaction under a condition of neutral pH. Further, any of the foregoing deaminoneuraminidases is an enzyme which originates from the animal. No deaminoneuraminidase has been known at all, which originates from any microorganism. Accordingly, an obtainable amount of the enzyme has been also limited.